1. Field of the Invention
The invention relates to a membrane filtration method and an apparatus for continuously monitoring the state of a membrane during the filtration. In particular, the present invention relates to a method and an apparatus for continuously monitoring for membrane fouling due to cake or gel layers of solutes developed on the surfaces of a filtered membrane.
2. Description of the Related Art
Membrane potential is based on the principles of electrostatics and electrokinetics. Membrane potential can provide useful information on physicochemical characteristics without disturbing the membrane filtration under the actual operating states or conditions. Solute rejection is an important factor indicating the filtration performance of a membrane, which can be determined by comparing the concentrations of a feed and a permeate.
Both the methods and the apparatus relating to the determination of membrane potential and membrane fouling are known in the art. According to K. J. Kim et al., Journal of Membrane Science, 134;199-208 (1997), the zeta potentials of virgin and fouled membranes were measured and a comparison based on the spectroscopic analysis was performed to investigate the changes of membrane potentials due to the membrane fouling. This article established that membrane potentials were altered before and after filtration through the intermittent measurements of membrane potentials of the fouled membranes at certain intervals. However, this paper has a disadvantage in that it cannot continuously confirm the behavior of the change of membrane potentials that are directly represented in the progress of membrane filtration under variable conditions. Consequently, in order to observe changes over the filtration time in membrane potentials, several identical membranes should be prepared and membrane potentials should be measured at specific intervals during the membrane filtration process. As a result, it is difficult to apply the results from Kim et al. to the real successive operating processes. In addition, a large number of membranes are actually required.
Journal of Membrane Science, 87:245-256, Elsevier (1994), discloses an apparatus which is capable of measuring simultaneously both membrane potentials and permeate fluxes. Flux measurements can be made simultaneously with the measurements of membrane potential. However, this apparatus is limited in that the correlation between solute rejections, membrane potentials, and permeate fluxes, cannot be identified. In particular, it is not possible to simultaneously monitor the solute rejections and the membrane potentials and, thus, impossible to identify the correlation between the solute rejections and the membrane potentials.
German Patent No. 3840901 discloses a membrane cell wherein horizontal channel flows are made on the surfaces of membranes in order to measure their zeta potentials. Such membrane cells do not permit for the membrane filtration and measurement of the zeta potentials to be conducted simultaneously. Also, as previously noted, the known membrane cells are not capable of monitoring the changes in characterization of the membrane over the passage of time.
Japanese Patent No. 8-101158 discloses a method for measuring streaming potentials associated with porous materials. Japanese Patent No. 10-38836 describes an apparatus for measuring streaming potentials. Both references show that the measurement of membrane potential cannot be performed simultaneously and continuously with the membrane filtration under specific operating pressure because the vessel for receiving the permeate is closed.
In order to solve the above-mentioned problems in the art, it is an object of the present invention to provide both a method and an apparatus which are capable of conducting membrane filtration simultaneously with the continuous measuring of potentials that are caused by minute differences in pressures between the upper and the lower surfaces of a membrane while also monitoring solute rejections due to variations in the concentration of the solution.
It is a further object to provide both a method and an apparatus that are capable of monitoring the progress of membrane fouling by continuously measuring the permeate flux, the membrane potential, and the solute rejection.
In order to accomplish these objects, the present invention provides for a membrane filtration method and an apparatus that continuously monitors for membrane fouling and changes in the membrane performance by simultaneously measuring the zeta potential of the membrane, solute rejection determined from the changes in the concentration, and the permeate flux. Essentially, the measurement of membrane potential according to the present invention is carried out by measuring streaming potentials originating from the inside of membrane pores and using the electrodes situated at both sides of the membrane while macromolecules, biopolymers, and inorganic aqueous solutions are filtered through the membrane.
The apparatus according to the present invention comprises a feed tank for receiving feed, a membrane module having an upper membrane cell and a lower membrane cell, a membrane situated between the upper and lower membrane cells, means for measuring properties of the feed, means for supplying the feed from the feed tank to the upper membrane cell of the membrane module, means for measuring a difference in the pressures between an upstream side and a downstream side of the membrane, means for measuring a streaming potential which builds inside of membrane pores, means for measuring concentration of the permeate which permeates through the membrane, and means for determining membrane fouling by estimating membrane potential using the physical properties of the feed, variations in the pressure difference between the upstream and downstream sides of the membrane and variations of the streaming potential difference, and estimating solute rejection of the membrane using the concentration difference between the feed and the permeate.
The apparatus further comprises means for measuring an amount of the permeate which is discharged from the lower membrane cell to obtain a permeate flux of the membrane.
The method for continuously monitoring the progress of membrane fouling over the filtration time using the apparatus of the present invention, comprises the steps of measuring properties of the feed that is fed from the feed tank to the upper membrane cell, measuring a difference in the pressures between the upstream side and the downstream side adjacent to the membrane, measuring a streaming potential developed inside of membrane pores, and obtaining a membrane potential using variation of the measured streaming potential, the properties of the feed and variation of the difference in the pressures between the upstream and the downstream sides of the membrane, measuring a concentration of the permeate which is passed through the membrane, and obtaining solute rejection of the membrane using measured concentration of the permeate and the concentration of the feed, and determining whether there is membrane fouling by simultaneously and continuously monitoring the estimated solute rejection and membrane potential.
The method further comprises the step of obtaining a permeate flux by measuring an amount of increase in the permeate per unit time. The present invention is described in more detail with reference to the accompanying drawings.